Low-emissions industrial burner

ABSTRACT

A burner ( 10 ) for use in both high O 2  environments and low O 2  environments comprises an outer tube ( 24 ) and an inner tube ( 26 ). The outer tube ( 24 ) defines a flow passage ( 53 ) and includes an inlet portion ( 42 ), an outlet portion ( 46 ), and a nozzle portion ( 44 ) interconnecting the inlet portion ( 42 ) and outlet portion ( 46 ). The inlet portion ( 42 ) has a larger effective cross-sectional area than the outlet portion ( 46 ) so that air ( 20 ) or an air-and-fuel mixture ( 35 ) moving through nozzle portion ( 44 ) is accelerated. The inner tube ( 26 ) is positioned to lie in the flow passage ( 53 ) of the outer tube ( 24 ) and is formed to include fuel-injection holes ( 78 ) to conduct fuel ( 33 ) into the flow passage ( 53 ).

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national application of internationalapplication Ser. No. PCT/US98/09525 filed May 13, 1998, which claimspriority to U.S. provisional applications Ser. Nos. 60/046,358 and60/077,926 filed May 13, 1997, and Mar. 13, 1998, respectively.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to burner assemblies, and particularly, toa low-emissions industrial burner. More particularly, the presentinvention relates to a low-emissions industrial burner for burning acombustible mixture to produce a flame.

One challenge facing the burner industry is to design a burner withminimal parts that produces low nitrogen oxide (NO_(x)) emissions duringoperation. Typically, a mixture of gaseous fuel and either air or oxygenin the proper ratio is created in an industrial burner to produce acombustible fuel-and-air mixture. The mixture is then ignited and burnedto produce a flame that can be used to heat various products in a widevariety of industrial applications. However, when the fuel and air arenot mixed completely or are not mixed in the proper ratio, combustion offuel such as natural gas, oil, liquid propane gas, low BTU gases, andpulverized coals often produce high levels of several unwanted pollutantemissions such as nitrogen oxide (NO_(x)), carbon monoxide (CO), andtotal hydrocarbons (THC).

According to the present invention, a burner is provided having an outertube defining a flow passage and an inner tube being positioned to liein the flow passage. The outer tube includes an inlet portion having alarge diameter, an outlet portion having a small diameter that issmaller than the large diameter of the inlet portion, and a nozzleportion interconnecting the inlet and outlet portions. The inlet portionof the outer tube is adapted to be coupled to an air supply to conductair through the flow passage. The inner tube includes an inlet end thatis adapted to be coupled to a fuel supply and is formed to include atleast one fuel-injection hole to conduct fuel from the fuel supply intothe flow passage to establish a combustible air-and-fuel mixture withinthe flow passage.

In one preferred embodiment, the burner is coupled to a long refractoryblock. The long refractory block extends beyond the outlet end of theburner and creates a flame chamber within the refractory block forcontaining the flame. The fuel-injection holes formed in the inner tubeare preferably positioned in the outlet portion of the outer tube.However, the fuel-injection holes can also be positioned in the nozzleportion or inlet portion of the outer tube and/or an air-and-fuelmixture can be supplied at the inlet end of the burner.

In a second embodiment, a burner is coupled to a short refractory block.The short refractory block terminates prior to the outlet end of theburner so that the outlet end of the burner extends beyond therefractory block. This allows an air-and-fuel mixture discharged from anexit end of the burner to mix with recirculated furnace gas contained ina furnace chamber in which the flame burns because the flame is notcontained within a flame chamber defined by the refractory block.

Additional features of the present invention will become apparent tothose of ordinary skill of the art upon consideration of the followingdetailed description of preferred embodiments exemplifying the best modeof carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figuresin which:

FIG. 1 is a side elevation view of a burner assembly including a burnerin accordance with a first embodiment of the present invention, withportions broken away, showing a fuel supply and air supply coupled to aninlet end of the burner and a long refractory block coupled to an outletend of the burner and formed to include a chamber for containing a flameproduced by the burner, the burner including an outer tube carrying aswirl plate and conducting air discharged from the air supply andswirled by the swirl plate through a nozzle section to the flamechamber, an inner tube coupled to the fuel supply and configured todischarge fuel into an accelerated air stream in the outer tube so as tocreate a combustible air-and-fuel mixture therein, a bluff-body flameholder coupled to a downstream end of the inner tube, and an ignitorcoupled to the outer tube to ignite the combustible air-and-fuel mixturetherein;

FIG. 2 is a view of the burner assembly taken along lines 2—2 of FIG. 1through the outer tube at a location downstream of the swirl plateshowing the inner fuel tube extending through the outer air tube;

FIG. 3 is a front perspective view of the burner of FIG. 1 showing thebluff-body flame holder positioned in the flame chamber formed in therefractory block;

FIG. 4 is a perspective view of the inner tube of FIG. 1 showing thebluff-body flame holder coupled to the downstream end of the inner tubeand fuel-injection holes formed in a portion of the inner tube locatedupstream from the bluff-body flame holder;

FIG. 5 is a rear perspective view of the burner of FIG. 1 showing theswirl plate coupled to an air inlet section of the burner for swirlingthe air flow through the burner and showing a fuel supply tube extendingperpendicularly through the outer tube in the air inlet section of theburner,

FIG. 6 is a side elevation view of a burner similar to the burner ofFIG. 1, showing placement of fuel-injection holes in an inner tube at alocation lying downstream from the location shown in FIG. 1 and closerto the bluff-body flame holder to discharge fuel into a regionimmediately upstream of the bluff-body flame holder;

FIG. 6A is a perspective view of the inner tube of FIG. 6 showing thebluff-body flame holder coupled to the downstream end of the inner tubeand fuel-injection holes formed in a portion of the inner tube locatedimmediately upstream from the bluff-body flame holder;

FIG. 7 is a side elevation view of a burner similar to the burner ofFIG. 6, showing placement of fuel-injection holes in an inner tube at alocation lying upstream from the location shown in FIG. 6 and closer tothe swirl plate to discharge fuel into a region immediately downstreamof the swirler;

FIG. 7A is a perspective view of the inner tube of FIG. 7 showing thebluff-body flame holder coupled to the downstream end of the inner tubeand fuel-injection holes formed in a portion of the inner tube locatedat the upstream end of the inner tube;

FIG. 8 is a side elevation view of a burner similar to the burners ofFIGS. 1-7 in accordance with another embodiment of the present inventionshowing admission of a premixed air-and-fuel mixture into the inlet endof the burner with no fuel supply coupled to the inner tube;

FIG. 9 is a side elevation view of a burner similar to the burner ofFIG. 8 in accordance with yet another embodiment of the presentinvention showing admission of a premixed air-and-fuel mixture into theinlet end of the burner in combination with a fuel supply coupled to theinner tube to allow fuel from the fuel supply to be discharged throughfuel-injection holes formed in the inner tube and combined with theair-and-fuel mixture admitted through the inlet end of the burner;

FIG. 10 is a side elevation view of a burner similar to the burners ofFIGS. 1-9 in accordance with still another embodiment of the presentinvention showing injection of fuel through a fuel supply manifoldmounted at the inlet end of the burner;

FIG. 10A is a perspective view of the burner of FIG. 10 showing passageof fuel into the outer tube of the burner through circumferentiallyspaced-apart tubular spokes included in the wagon wheel-shaped fuelsupply manifold;

FIG. 11 is a perspective view of a burner in accordance with a furtherembodiment of the present invention showing an air-receiving passagewayhaving a somewhat rectangle-shaped cross-section;

FIG. 11A is a perspective view of a line burner in accordance with anadditional embodiment of the invention showing three burners of the typeshown in FIG. 11 arranged in sequence to define a line burner assembly;

FIG. 12 is a perspective view of a burner similar to the burner in FIGS.1-5, with portions broken away, showing an inner fuel supply tubeconfigured to discharge fuel from a primary fuel supply through fuelinjection holes formed in the inner tube and a pair of concentric tubesextending through the inlet end of the burner and into the inner tubeand terminating at the flame holder, an outer tube of the concentrictubes being configured to discharge oxygen from an oxygen supply at anoutlet end of the burner and an inner tube of the concentric tubes beingconfigured to discharge fuel from a secondary fuel supply at a flameoutlet end of the burner;

FIG. 13 is a perspective view of a burner similar to the burner in FIGS.1-5, with portions broken away, showing an inner fuel supply tubeconfigured to discharge fuel from a primary fuel supply through fuelinjection holes formed in the inner tube and a single tube extendingthrough the inlet end of the burner an into the inner tube andterminating at the bluff-body flame holder for discharging waste gasfrom a waste-gas supply at a flame outlet end of the burner;

FIG. 14 is a side elevation view of a burner assembly similar to theburner assembly of FIGS. 1-5, showing the burner of FIGS. 1-5 beingcoupled to a furnace chamber using a short refractory block so that anair-and-fuel mixture discharged from an exit end of the burner mixeswith recirculated furnace gases (products of combustion) contained inthe furnace chamber;

FIG. 15 is an exploded side elevation view of the burner assembly ofFIG. 14, showing burner of FIG. 14 in more detail; and

FIG. 16 is a side elevation view of a burner assembly similar to theburner assembly in FIG. 15, showing the burner without a swirler.

DETAILED DESCRIPTION OF THE DRAWINGS

A burner in accordance with the present invention is well-suited for usein high-oxygen processes or environments such as thermal oxidizers, fameincinerators, and pollutant-burning afterburners wherein theconcentration of oxygen (O₂) in the process chamber is greater thantwelve percent (typically seventeen to nineteen percent oxygen). Thepresent burner is also well-suited for use in low-oxygen processes orenvironments such as boilers, furnaces, kilns, and rotary dryers whereinthe concentration of oxygen in the process chamber is less than or equalto twelve percent (typically less than six percent). Burner 10 can alsobe used, for example, to incinerate industrial fumes, to heat water, orto generate steam.

A burner assembly 11 including a burner 10 in accordance with thepresent invention is illustrated in FIG. 1. Burner 10 operates inconjunction with an air supply 12, a fuel supply 14, and a refractoryblock 16 to produce a low-emissions flame 18 within a flame chamber 19formed in refractory block 16. Burner 10 includes an outer tube 24, aninner tube 26, a swirler 28, an ignitor 30, and a bluff-body flameholder 32. As used herein, “tube” means any conduit or channel,regardless of shape (i.e., cylindrical cross-section, rectangularcross-section or otherwise), through which something (such as a liquid,solid, or gas) is conveyed or conducted. Swirler 28 is positioned to lieat an air inlet end 36 of burner 10 and flame holder 32 is positioned tolie at a flame outlet end 38 of burner 10. Inner and outer tubes 26, 24and flame holder 32 are preferably made of heat resistant alloys. Forexample, inner and outer tubes 26, 24 are preferably made of 18-8stainless steel, and bluff-body flame holder 32 is made of 310 stainlesssteel.

In use, air 20 is introduced on a centerline 82 of burner 10 into a rearportion of burner 10 and directed to pass over swirler 28, whichprovides stability when excess air is present, and mix with gaseous fuel33, which is injected perpendicularly to the stream of air 20. Acombustible air-and-fuel premixture 35 is thus established, which flowsthrough a smooth flow passageway 73 and passes over bluff-body flameholder 32, where premixture 35 burns within refractory block 16 (or, forexample, a metallic sleeve) to produce flame 18.

Burner assembly 11 uses refractory block 16 as a combustion chamber inconjunction with an air-and-fuel premixing apparatus which operates topremix air 20 and fuel 33 partially prior to ignition while the air 20is forced through burner 10 by a fan (not shown) coupled to air supply12. Ignitor 30 is positioned to communicate with air-and-fuel premixture35 at a point in burner 10 upstream of the zone of flame attachment. Inburner 10, the possibility of early flame attachment is minimizedbecause: (1) air-and-fuel mixture 35 is moved at a velocity that exceedsthe flame speed and (2) the flow passageway 75 is relatively smooth tominimize possible turbulence. Although the burner of the presentinvention includes both of these features, either feature alone (as wellas other features) could be used to accomplish the same result. Forexample, even if a burner does not have a “smooth” flow passage, themixture could be moved at a high enough velocity to avoid early flameattachment. Similarly, an extremely smooth flow passage could be usedeven with lower mixture velocities while avoiding early flameattachment. Thus, although both features are present in the presentpreferred embodiment, it is within the scope of the present invention tominimize the possibility of early flame attachment using either featureindependently or other similar features.

As shown in FIG. 1, in the preferred embodiment burner 10 has foursections: an air-admitting section (or inlet portion) 42, anair-accelerating section (or nozzle portion) 44, a mixing/ignitingsection (or outlet portion) 46, and a flame-holding section 48. Outertube 24 is shaped and configured to define sections 42, 44, and 46.Bluff-body flame holder 32 is positioned to lie adjacent to one end ofouter tube 24 to define section 48.

Air-admitting section 42 of burner 10 is defined by a cylindricalportion of outer tube 24 located at air inlet end 36 of burner 10 asshown, for example, in FIG. 1. This inlet portion 42 of outer tube 24has a relatively larger inner diameter 52 and defines a low-velocitypassageway 54 that conducts swirling air 20 discharged from air supply12 and passed through swirler 28 in a downstream direction 43 towardair-accelerating section (or nozzle portion) 44. Air 20 passing throughlarge-diameter passageway 54 in air-admitting section 42 travels indownstream direction 43 at a relatively low velocity, thereby minimizingthe air pressure drop across swirler 28. Preferably, air 20 travels at avelocity approximately equal to 50 feet/sec (1524 cm/sec) withinair-admitting section 42 which results in a pressure drop of about 0.5inches of water (column) (12.70 Kg./sq. meter) across swirler 28. An airpressure tap 83 is coupled to outer tube 24 and configured to sensepressure of air 20 in passageway 54. By locating swirler 28 in alow-velocity environment away from ignitor 30 and refractory block 16,swirler 28 is less likely to be damaged by heat or high-velocitypressures and therefore is likely to last longer.

Air-accelerating section 44 of burner 10 is defined by a conical portionof outer tube 24 located between air inlet end 36 and flame outlet end38 of burner 10 as shown, for example, in FIG. 1. Conical (or nozzle)portion 44 has an inner diameter 52 at its inlet end 62, a relativelysmaller inner diameter 70 at its outlet end 66, and a nozzle-shapedpassageway 65 that converges in downstream direction 43. Nozzle-shapedpassageway 65 functions like a nozzle to accelerate the flow rate of air20 flowing from air-admitting section 42 through air-acceleratingsection 44 toward flame outlet end 38 of burner 10. When swirler 28 ismounted in chamber 54 of air-admitting admitting section 42, then air 20passing through nozzle-shaped passageway 65 is swirling while it isaccelerating.

Mixing/igniting section (or outlet portion) 46 of burner 10 is definedby a cylindrical portion of outer tube 24 located at flame outlet end 38of burner 10 as shown, for example, in FIG. 1. Cylindrical outletportion 46 of outer tube 24 has a smaller inner diameter 70 and conductsaccelerated, swirling air 20 discharged from air-accelerating section 44at high velocity further along in downstream direction 43 toward flamechamber 19 in refractory body 16. Inner tube 26 is configured todischarge fuel 33 into the accelerated, swirling air 20 passing throughcylindrical portion 46 of outer tube 24 so that a combustibleair-and-fuel mixture 35 moves at high velocity past ignitor 30 towardflame chamber 19.

Inner tube 26 is positioned to lie in outer tube 24 as shown, forexample, in FIG. 1 and is formed to include constant outer diameter 71.An upstream end 25 of inner tube 26 is coupled to fuel supply 14 by afuel supply line 27 and a downstream end 29 of inner tube 26 isconfigured to support bluff-body flame holder 32 in flame chamber 19 ofrefractory block 16 in spaced-apart relation to a downstream end 31 ofthe cylindrical portion 46 of outer tube 24. Fuel supply line 27includes an elbow-shaped pipe 58 coupled to upstream end 25 of innertube 26 and a supply pipe 56 coupled to elbow-shaped pipe 58 and to fuelsupply 14. Supply pipe 56 passes through an opening 57 formed in a sidewall of outer tube 24 as shown, for example, in FIGS. 1-4. A pilot inlettube 81 is appended to supply pipe 56 as shown, for example, in FIGS.1-14.

Inner tube 26 is configured to conduct fuel 33 received from fuel supplyline 27 through a passageway 77 formed therein as shown, for example, inFIG. 4 and then discharge fuel 33 into outer tube 24 so that it mixeswith swirling air 20 conducted through cylindrical portion 46 of outertube 24 to form a combustible air-and-fuel mixture 35 traveling indownstream direction 43 through a high-velocity passageway 73 defined byinner and outer tubes 26, 24 toward bluff-body flame holder 32 and flamechamber 19 in refractory body 16. In a preferred embodiment,high-velocity passageway 73 is annular and surrounds a cylindricalexterior surface 39 of inner tube 26 and is bounded by a cylindricalinterior surface 37 of outer tube 26 that is positioned to surroundinner tube 26.

Air 20 has accelerated to a maximum velocity at exit end 66 ofair-accelerating section 44 and then enters inlet end 74 ofhigh-velocity passageway 73 provided in mixing/igniting section 46. Air20 continues to flow and swirl through the mixing/igniting section 44 ata constant velocity because inner diameter 70 of high-velocitypassageway 73 remains constant along the length of mixing/ignitingsection 46. The distance between inner tube 26 and outer tube 24 withinthe mixing/igniting section 46 is shown as constant radial gap 72 thatdefines annular high-velocity passageway 73 in mixing/igniting section46. The axial airspeed through the mixing/igniting section 44 should beslow enough to allow thorough mixing of the fuel and air, but fastenough to prevent early flame attachment (i.e., flame attachmentupstream from the flame holder). For example, airspeed of 250feet/second (7620 cm/sec) has been found to be slow enough for completemixing but fast enough to avoid early flame attachment for burnershaving a turndown ratio of 15:1.

Fuel-injection holes 78 are formed in inner tube 26 at a point nearinlet end 74 of high-velocity passageway 73 in mixing/igniting section46 in the embodiment of FIGS. 1-5 to communicate with thefuel-conducting passageway 77 formed in inner tube 26 so that fuel 33discharged from passageway 77 in inner tube 26 is injectedperpendicularly into high-velocity, swirling air 20 discharged fromnozzle-shaped passageway 65 in air-accelerating section 44 of burner 10.The distance 80 from fuel-injection holes 78 to flame holder 32 iscalled the “mixing length” and is preferably two times the hydraulicdiameter, where the hydraulic diameter equals the inner diameter 70 ofhigh-velocity passageway 73 minus the outer diameter 71 of inner tube26. Perpendicular fuel injection into a stream of swirling air causesfuel 33 to mix with air 20 in a “complete” manner. By locatingfuel-injection holes 78 near inlet end 74 of high-velocity passageway 73after air 20 has been accelerated to its maximum velocity in burner 10,the chance of having fuel 33 flow upstream in direction 45 back towardsair-accelerating section 44 is minimized. Also, by injecting fuel 33into the accelerated air, the chance of burning within the burner 10 isminimized.

Fuel-injection holes 78 are positioned to lie in circumferentiallyspaced-apart relation to one another around cylindrical exterior surface39 of inner tube 26 so that the fuel-injection holes 78 are aligned tolie along a plane 47 that slices perpendicularly through inner tube 26,as shown in FIG. 1. Preferably, fuel is injected at a pressure of fourtimes air pressure and fuel-injection holes 78 are spaced approximately45° apart so that the proper amount of fuel 33 can be injected intohigh-velocity swirling air 20 in the proper stoichiometric ratio. Thecombination of mixing length 80, annular gap 72, and the diameter andspacing of fuel-injection holes 78 allows burner 10 to achieve lowNO_(x) emissions, given the proper air/fuel ratio.

Air-and-fuel mixture 35 travels toward exit end 76 of mixing/ignitingsection 46. Ignitor 30 ignites mixture 35 so that mixture 35 burnstemporarily within high-velocity passageway 73 in mixing/ignitingsection 46 of burner 10. However, ignitor 30 stays lit for less than 4seconds so that air-and-fuel mixture 35 will not continue to burn withinhigh-velocity passageway 73 of mixing/igniting section 46. Instead,because of acceleration of flow rate of swirling air 20 in nozzle-shapedpassageway 65 of air-accelerating section 44, the flow rate ofair-and-fuel mixture 35 is sufficiently high (i.e., greater than 0.25inches of water (column)) (6.35 Kg./sq. meter) so that the ignitedair-and-fuel mixture 35 is “pushed” downstream in direction 43 out ofmixing/igniting section 46 of burner 10 by unlit mixture once theignitor 30 is turned off.

After the ignited fuel-and-air mixture passes through exit end 76 ofmixing/igniting section 46, the ignited mixture 35 must pass aroundbluff-body flame holder 32 mounted on downstream end 29 of inner tube26. Preferably, bluff-body flame holder 32 is offset slightly by offsetdistance 49 (i.e., a distance less than the inner tube 26) from exit end76 of mixing/igniting section 46 so that bluff-body flame holder 32resides within the flame chamber 19 formed in refractory block 16, asshown in FIG. 1. This not only enhances mixing by allowing more air andfuel to flow out of exit end 76, but it also allows bluff-body flameholder 32 to be serviced easily since a wrench can be applied to aportion of inner tube 26 that extends in direction 43 past downstreamend 31 of outer tube 24 without interference from outer tube 24. Bypositioning bluff-body flame holder 32 away from the air-and-fuel mixingchamber in high-velocity passageway 73, a larger recirculation patterncan be achieved without having to introduce fuel 33 out to flame holder32 to stabilize flame 18 without a NO_(x) penalty.

Once the ignited air-and-fuel mixture 35 passes through exit end 76,flame 18 attaches to bluff-body flame holder 32 within flame chamber 19in refractory block 16 where it continues to burn. Preferably,refractory block 16 is made of alumina/silica, although other refractoryblock materials could also be used. In addition, burner 10 is capable ofbeing operated without using a refractory block 16 and still achieveslow NO_(x) emissions with low levels of excess air coming through theburner.

As shown in FIGS. 1-5, burner 10 is connected to refractory block 16 byfaceplate 90 and nuts and bolts 92, 94. Preferably, a sight glass 96 canalso be used to ensure that a proper flame 18 is burning withinrefractory block 16. Preferably, face plate 90 is continuously welded toouter tube 24 to ensure that no leakage occurs between faceplate 90 andouter tube 24.

As shown in FIG. 3, burner 10 and refractory block 16 are generallycylindrical in shape and are connected by generally circular face plate90. However, as shown in FIG. 5, burner 10 is also slightlyfunnel-shaped due to the nozzle-shaped configuration of air-acceleratingsection 44 located between the upstream air-admitting section 42 and thedownstream mixing/igniting section 46.

Air inlet end 36 of burner 10 is also shown best in FIG. 5. As shown inFIG. 5, swirler 28 includes fins 112 and a body portion 114 coupled tofins 112. Fins 112 extend radially outwardly from body portion 114 andare twisted in a fan-like manner so that air 20 from air supply 12enters air-admitting section 42 in a swirling manner as shown in FIG. 1.As mentioned above, by locating swirler 28 in a low-velocity environmentaway from ignitor 30 and refractory block 16, swirler 28 is less likelyto be damaged by heat or high pressures and therefore is likely to lastlonger.

Inner fuel tube 26 and bluff-body flame holder 32 are shown in moredetail in FIG. 4. Inner tube 26 is formed to include fuel-injectionholes 78 that are equally spaced around the circumference of inner tube26. Downstream from fuel-injection holes 78, bluff-body flame holder 32is attached to inner tube 26. Bluff-body flame holder 32 not only aidswith the mixing of air 20 and fuel (not shown), but bluff-body flameholder 32 also closes downstream end 29 of inner tube 26 so that fuel 33discharged into upstream end 25 of inner tube 26 from fuel supply line27 is forced to flow out of fuel-injection holes 78 to mix withhigh-velocity, swirling air 20 passing through annular high-velocitypassageway 73 surrounding inner tube 26 and communicating withfuel-injection holes 78.

Burner 10 swirls air 20 in a relatively low-velocity (large innerdiameter) passageway 54 downstream of swirler 28 to minimize pressuredrop across swirler 28. Burner 10 accelerates air 20 throughnozzle-shaped passageway 65 from low-velocity passageway 54 toward afuel-injection point (e.g., 78) to minimize the chance of fuel 33flowing in upstream direction 45 after it mixes with air 20 inhigh-velocity passageway 73. Fuel 33 is injected into air 20 movingthrough passageway 73 in burner 10 to avoid the need for creating anair-and-fuel premixture outside of burner 10. Fuel 33 is injected intofast-moving air 20 that had been accelerated in nozzle-shaped passageway65 to minimize chance of burning occurring inside burner 10. Fuel 33 isinjected perpendicularly to air stream 20 in a manner that providessufficient mixing to achieve low NO_(x) emissions, given the proper airand fuel ratio. Burner 10 stabilizes flame 18 (i.e., prevent flame 18from blowing out) in the swirling wake of bluff-body flame holder 32,which is positioned to lie a short distance 49 (preferably less than onethroat-pipe diameter, i.e., radial gap 72) inside the flame chamber 19formed in refractory block 16.

Burner 10 is used, for example, in the field of fume incineration. Forexample, when cars or trucks are processed through paint systems duringthe manufacturing process, burner 10 can be used to burn off the paintfumes instead of emitting the fumes into the atmosphere. Similarly,burner 10 can be used to burn petroleum fuel vapors that are createdwhen petroleum fuel is transferred from one process to another.Additionally, burner 10 can be used to burn fumes that are created bysemiconductor chip manufacturers during a chip manufacturing process.

Burner 10 can also be used for other applications in which a burner isnecessary. For example, burner 10 could be used to incinerate liquid orsolid waste products from almost any manufacturing process. In addition,burner 10 could be used to burn off waste products that are createdduring the manufacturing process of drywall material during a calciningprocess. Burner 10 could also be used in the furnace industry oraggregate dryer industry.

The burner 10 of the present invention can be configured to achieve lowNO_(x) emissions in both high O₂ environments and low O₂ environments.As mentioned above, a high O₂ environment is one in which O₂ in theprocess chamber (or furnace chamber) is greater than 12% (typically17-19%). In this environment, the burner 10 can only achieve low NO_(x)emissions by operating in excess air mode. Further, a swirler is neededto operate in excess air mode. Accordingly, although burner 10 can runwith or without a swirler 28, the swirler 28 must be included in theburner 10 to achieve low NO_(x) emissions in the high O₂ environment.Although swirler 28 is not needed for burner 10 to operate, swirler 28creates a “slow” area in the middle of the flame that resembles an “eyeof the storm” and this helps stabilize flame 18 generated by burner 10.To achieve low NO_(x) emissions, in a low O₂ environment (i.e., whereinthe O₂ in the process chamber is less than or equal to 12%—typicallyless than 6%), a short refractory block 116 is used in conjunction withburner 10 to achieve low NO_(x) emissions and a swirler is not needed.This embodiment is described in more detail below with reference toFIGS. 14-16.

Burner 10 incorporates mixing techniques that provide for a desirablyshort burner length. The gas 33 is injected perpendicular to the air 20with a momentum flux that optimizes mixing within an annulus 73. Thenthe gas 33 and air 20 exit the annulus 73 and pass over the flame holder32 prior to burning. Because the flame holder 32 stands off the annulus73, the gas 33 and air 20 have further time to mix after exiting theannulus 73. The post-exit area is larger than the annulus 73, whichmeans that the flow 35 decelerates as it exits. Shear forces created bythis deceleration as well as the changes in velocity directlyattributable to the flame holder 32 itself mix the gas 33 and air 20prior to combustion. Enhanced mixing reduces emissions from burner 10.Standing the flame holder 32 off from the annulus 73 requires carefulattention to velocities to prevent burning behind the flame holder 32but allows for very quick mixing.

Fuel-injection holes 78 can be formed at any point in inner tube 26 asshown, for example, in the embodiments of FIGS. 6 and 7, to enabledischarge of fuel 33 conducted through passageway 77 in inner tube 26into high-velocity passageway 73 formed in mixing/igniting section 46.While fuel-injection holes 78 are formed in inner tube 26 at a pointnear inlet end 74 of high-velocity passageway 73 in the embodiment ofFIG. 1, fuel-injection holes 78 are moved in a downstream direction 43in the embodiment of FIG. 6 so as to be formed in inner tube 26 at apoint near exit end 76 of high-velocity passageway 73. However, in theembodiment of FIG. 7, the fuel-injection holes 78 are moved in anupstream direction 45 and are formed in a section of inner tube 26positioned to lie in air-admitting section 42 (low-velocity passageway54) near the swirler 28. Fuel-injection holes 78 could also be formed ina section of an inner tube 26 positioned to lie in air-acceleratingsection 44 (nozzle-shaped passageway 65).

In the embodiments shown in FIGS. 1-7, outer tube 24 of burner 10 isformed to include a single “axi-symmetric” flow passage 53 defined bypassageways 54, 65, 73 that admits air 20 in a large-diameter passageway54 which minimizes the pressure drop across swirler 28 or otherobstructions at that location. Because the outer tube 24 is preferablycylindrical, the differences in diameter between inlet portion 42,outlet portion 46, and nozzle portion 44 determine how the air 20 ormixture 35 is accelerated. However, as shown in FIG. 11, when the shapeof the outer tube 24 is something other than cylindrical, thedifferences in effective cross-sectional areas of the inlet portion 42,outlet 35 portion 46, and nozzle portion 44 determine the acceleration.Thus, the differences in effective cross-sectional area between theseportions 42, 44, 46 determine the acceleration of air 20 or mixture 35for cylindrical cross sections as well as other cross sections.

In the embodiments of FIGS. 1-6, air 20 then accelerates toward ignitor30 through conical portion 44. This eliminates the chance of having anupstream flow of fuel and improves air flow distribution if the inletair flow is unbalanced. Fuel 33 is then injected into air 20 at anypoint within the high-velocity passageway 73 (FIGS. 1-6), to minimizethe potential for unwanted early ignition that could result if apremixture was created in a pipe train leading to the burner. However,as fuel-injection holes 78 are moved in an upstream direction 43 fromthe position shown in FIGS. 1-5 to the position shown in FIG. 6, themixing length distance (from the fuel-injection holes 78 to the flameholder 32) is reduced from a distance 80 in FIGS. 1-5 to a distance 180in FIG. 6. Although the shorter distance 180 in FIG. 6 minimizes thechance of burning within burner 10, the larger distance 80 in FIGS. 1-5is preferable because more “complete” mixing can be accomplished. Thefuel 33 is injected into the air 20 perpendicularly to cause fuel 33 tomix with air 20 in a “complete” manner. Finally, a bluff-body flameholder 32 stabilizes flame 19 within the combustion chamber 19.

As shown in FIG. 7, by locating the fuel-injection holes 78 immediatelydownstream of the swirler 28 in the low-velocity passageway 54 formed inair-admitting section 44, fuel 33 can be injected into a region withinburner 10 containing a highly turbulent, low-velocity air flow (ascompared to the air flow in the high-velocity passageway 73 shown in theembodiments of FIGS. 1-5 and 6). The mixing length distance 280 betweenfuel-injection holes 78 and flame holder 32 in the embodiment of FIG. 7is longer than the mixing length distances 80, 180 shown in theembodiments of FIGS. 1-5 and 6, respectively, to facilitate mixing ofair and fuel in the burner. Although injection of fuel 33 downstreamfrom swirler 28 reduces the mixing that would be gained by having themixture pass through swirler 28, this downstream fuel injection withinpassageway 73, 54, or 65 ensures that the swirler 28 will not get burnedup, especially at lower flow rates.

In the embodiment of FIG. 8, there are no fuel-injection holes 78 formedin inner tube 26 and there is no fuel supply coupled to the inner tube26. Instead, the inner tube 26 simply acts as a support for the flameholder 32. As shown in FIG. 8, a premixed air-and-fuel mixture 220 isadmitted into the inlet end 36 of the burner 10. The air-and-fuelmixture 220 passes through swirler 28 as it enters air-admitting section42. The air-and-fuel mixture 220 is then accelerated through conicalportion 44 before entering cylindrical portion 46. The air-and-fuelmixture 220 is then ignited within the cylindrical portion 46 and isforced in the downstream direction 43 to produce a flame (not shown)that attaches to flame holder 32.

In the embodiment of FIG. 9, the burner of FIG. 8 is modified so thatinner tube 26 is formed to include injection holes 78 and is coupled tofuel supply 14. Although injection holes 78 are shown at a location nearflame holder 32, the position of fuel injection holes 78 can be in anyof the positions shown in FIGS. 1-7 or the description relating to FIGS.1-7. Thus, in the embodiment of FIG. 9, the air-and-fuel mixture 220 canbe supplemented by having fuel 33 injected through holes 78 withinpassageways 54, 65, or 73. Of course, the air-fuel ratio of theair-and-fuel mixture 220 coming in via the inlet end 36 need not be thesame as that coming in via the fuel-injection holes 78.

In the embodiment of FIG. 10, a fuel-injection manifold 260 is used toinject fuel 33 from fuel supply 14 into the burner 10 at the air inletend 36. As shown in FIG. 10A, manifold 260 includes a ring 262 definingan air-flow passageway 264 and a plurality of spoke-like injector tubes266 for discharging fuel 33 through apertures 265 formed in ring 262into the low-velocity air 20 passing through the air-flow passageway264. The injector tubes 266 may or may not be configured to induce swirlof air 20 passing from air supply 12 through spaces between the injectortubes 266 and outside of ring 262. The fuel-injection manifold 260 inaccordance with this embodiment is configured to inject fuel 33 throughtubes or other suitable injectors at the air inlet of the burner,thereby maximizing the mixing time of air and fuel within the burnerbody. A fuel-injection manifold in accordance with this embodiment iswell-suited for use with liquid fuels. In addition, although not shownin FIG. 10, a secondary fuel supply could be coupled to the inner tube26 with the inner tube 26 being formed to include fuel-injection holes78 as shown in FIGS. 1-7.

In the embodiment of FIG. 11, the burner 10 may incorporate any of thefuel-injection methods described above for FIGS. 1-10. However, in theembodiment of FIG. 11, the passageways 54, 65, 73 are rectilinear ratherthan axi-symmetric as shown in FIGS. 1-10. As shown in FIG. 11, asupplemental fuel-injection tube 226 is preferably used to inject fuel33 into air 20. The supplemental tube 226 is coupled to fuel supply 14and arranged to extend perpendicularly through inner tube 26 as shown inFIG. 11 so that fuel 33 will be distributed evenly throughout therectilinear sections 42, 44, 46. In this embodiment, the fuel-injectionholes 78 are formed on the supplemental tube 226 instead of the innertube 26. This embodiment could also be extended to cover a tee, a cross,an H, an I, or other suitable shape. Inner tube 26 is also used tosupport flame holder 32. Two or more rectilinear burners can be arrangedin line as shown in FIG. 11 A to create a line burner assembly 211supplied with fuel via supplemental tube 226 coupled to fuel supply 14.

In the embodiment of FIG. 12, a pair of concentric tubes 280, 282 areused to discharge secondary fuel 286 and oxygen 288 at the flame holder32. Preferably the oxygen 288 is 75% purity or higher, but oxygenpurities of less than 75% can also be used. The secondary fuel 286 andoxygen 288 travel through tubes 280 and 282 respectively so that thesecondary fuel 286 and oxygen 288 can burn at the face of the bluff-body flame holder 32 with or without the assistance of fuel 33 beingadmitted through fuel-injection holes 78, which can be located on any ofthe positions shown in FIGS. 1-7. A primary fuel supply tube 126 isconfigured to discharge fuel 33 from primary fuel supply 14 at the flameoutlet end of the burner. The concentric tubes 280, 282 extend throughthe inlet end of the burner and a portion of the primary fuel supplytube 126 and terminate at flame holder 32.

In the embodiment of FIG. 13, difficult-to-burn gas 92 such as asecondary gas or a waste gas is introduced through a single tube (orlance) 296. The embodiment of FIG. 13 is identical to the embodiment ofFIG. 12 except that only a single tube 296 is used. A primary fuelsupply tube 126 is configured to discharge fuel 33 from primary fuelsupply 14 at the flame outlet end of the burner. The waste-gas tube 296extends through the inlet end of the burner and a portion of the primaryfuel supply tube 126 and terminate at flame holder 32.

In the embodiments of FIGS. 14-16, a short refractory block 116 is used.Any of the burners shown in FIGS. 1-13 can be combined with the shortrefractory block 116 shown in FIGS. 14-16. As shown in FIG. 14, burner10 can be connected to a furnace chamber 17 using the short refractoryblock 116. With a short refractory block 116, the air-and-fuelpremixture 35 enters the furnace chamber 17 immediately upon exiting theexit end 76 of the cylindrical portion 46. The premixture 35 then mixeswith furnace gases 240 as the mixture 35 passes around flame holder 32.Furnace gases 240 are those gases that exist in a furnace, or otherprocess chamber, that are the by-products of fuel combustion—these gasescontain nitrogen, water vapor, carbon dioxide and the excess oxygen leftover from the combustion of the fuel. The momentum and viscosity of thepremixture 35 induces a circulating flow of furnace gases 240 within thecombustion chamber 17. The furnace gases 240 are entrained into thepremixture 35 so that the presence of the furnace gas 240 into thepremixture 35 dilutes the O₂ within the premixture 35 and adds to itsthermal capacitance. This reduces the adiabatic flame temperature whichultimately reduces the thermal NO_(x) formation rate. The furnace gas240 continues to migrate towards the premixture 35 across a diffusionboundary 241 between furnace gas 240 and premixture 35 so that furnacegas 240 is continually recirculated towards the flame 18. Because theshort refractory block 116 allows furnace gas 240 to be recirculated andburned within the furnace chamber 17, additional piping for externalfurnace gas recirculation is not needed. In addition, fuel staging isnot needed with the burner of the present invention in either the low O₂or the high O₂ environment. Also, in the low O₂ environmentanti-flashback mechanisms are not needed because fuel comes in at onlyone place such that secondary fuel supplied downstream for typical lowO₂ environments is not needed.

The burner of FIG. 14 is shown in more detail in FIG. 15. As shown inFIG. 15, the burner can be any of the burners shown in FIGS. 1-13 withthe exception that a short refractory block 116 is used. Similarly, FIG.16 shows that any of the burners of FIGS. 1-15 can be configured withouta swirler. All embodiments thus far have shown a swirler or some otherobstruction in the air inlet. Such an obstruction improves flamestability when the burner is run with excess air, however, the swirleror forms of obstruction are not required. Under this scenario, the flameis stable near Stoichiometric air/fuel ratios which is more likelyapplicable for the low O₂ environment.

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe scope and spirit of the invention as described and defined in thefollowing claims.

What is claimed is:
 1. A burner comprising an outer tube defining a flowpassage and including a large-diameter inlet portion having a largediameter, a small-diameter outlet portion having a small diameter beingsmaller than the large diameter of the large-diameter inlet portion, anda nozzle portion interconnecting an outlet end of the inlet portion andan inlet end of the outlet portion and converging toward the outletportion to establish the flow passage, an inlet end of thelarge-diameter inlet portion being adapted to be coupled to an airsupply to conduct air flowing through the flow passage to an outlet endof the outer tube and a small-diameter inner tube being positioned tolie in the flow passage of the outer tube, the inner tube having aninlet end positioned to lie in the flow passage outside of thesmall-diameter outlet portion and adapted to be coupled to a fuelsupply, the inner tube having a diameter that is smaller than the smalldiameter of the outer tube, and the inner tube being formed to include adownstream end opposite to the inlet end and a fuel-injection holepositioned to lie in spaced-apart relation to the downstream end toconduct fuel flowing from the inlet end of the inner tube through theinner tube into the air flowing through the flow passage to establish acombustible air-and-fuel mixture within the flow passage, and a flameholder coupled to the downstream end of the inner tube and positioned toextend beyond an exit end of the small-diameter outlet portion of theouter tube and lie outside the flow passage defined by the outer tube tocomplete the mixing of the fuel with the air as the combustibleair-and-fuel mixture exits the flow passage of the outlet end of theouter tube to produce a low-emission flame attached to the flame holderupon ignition of the combustible air-and-fuel mixture.
 2. The burner ofclaim 1, wherein the fuel-injection hole is formed in the inner tube sothat the hole is positioned to lie within the small-diameter outletportion of the outer tube.
 3. The burner of claim 2, further comprisingan ignitor coupled to the outer tube, positioned to lie between thefuel-injections hole and the flame holder, and configured to ignite thecombustible air-and-fuel mixture.
 4. The burner of claim 2, wherein aswirler is coupled to the outer tube within the large-diameter inletportion to swirl the air through the flow passage.
 5. The burner ofclaim 1, wherein the fuel injection hole is formed in the inner tube sothat the hole is positioned to lie within the nozzle portion of theouter tube.
 6. The burner of claim 5, further comprising an ignitorcoupled to the outer tube, positioned to lie between the fuel-injectionhole and the flame holder, and configured to ignite the combustibleair-and-fuel mixture.
 7. The burner of claim 5, wherein a swirler iscoupled to the outer tube within the large-diameter inlet portion toswirl the air through the flow passage.
 8. The burner of claim 1,wherein the fuel injection hole is formed in the inner tube so that thehole is positioned to lie within the large-diameter inlet portion of theouter tube.
 9. The burner of claim 8, further comprising an ignitorcoupled to the outer tube, positioned to lie between the fuel-injectionhole and the flame holder, and configured to ignite the combustibleair-and-fuel mixture.
 10. The burner of claim 8, wherein a swirler iscoupled to the outer tube within the large-diameter inlet portion toswirl the air through the flow passage.
 11. The burner of claim 1,wherein the inlet end of the large-diameter inlet portion is adapted tobe coupled to a fuel supply to conduct an air-and-fuel mixture throughthe flow passage.
 12. The burner of claim 1, wherein a fuel-injectionmanifold is adapted to be coupled to the inlet end of the large-diameterinlet portion of the outer tube, the manifold including a ring definingan air-flow passageway and at least one spoke-like injector tube fordischarging fuel through apertures formed in the ring into the airpassing through the air-flow passageway.
 13. A burner comprising anouter tube defining a flow passage and including a large-diameter inletportion having a large diameter, a small-diameter outlet portion havinga small diameter being smaller than the large diameter of thelarge-diameter inlet portion, and a nozzle portion interconnecting anoutlet end Of tile inlet portion and an inlet end of tile outlet portionto establish the flow passage, an inlet end of the large-diameter inletportion being adapted to be coupled to an air supply to conduct airflowing through the flow passage to an outlet end of the outer tube, aswirler is coupled to the outer tube within the large-diameter inlet toswirl air through the flow passage, means for discharging fuel into theair passing through the flow passage to form a combustible air-and-fuelmixture therein for discharge into a flame chamber at the outlet end ofthe small-diameter outlet portion wherein the means for discharging fuelincludes an inner tube having a diameter smaller than the diameter ofthe small diameter outlet portion of the outer tube, and a flame holdercoupled to a downstream end of the inner tube and positioned to lieoutside the flow passage of the outer tube.
 14. The burner of claim 13,wherein the inner tube has a fuel-injection hole formed therein fordischarging the fuel.
 15. The burner of claim 14, wherein thefuel-injection hole is positioned to lie within the small-diameterportion of the outer tube.
 16. The burner of claim 13, wherein the meansfor discharging fuel discharges fuel into the small-diameter outletportion of the outer tube.
 17. The burner of claim 16, wherein a swirleris coupled to the outer tube within the large-diameter inlet portion toswirl the air through the flow passage.
 18. The burner of claim 13,wherein the means for discharging fuel discharges fuel into the nozzleportion of the outer tube.
 19. The burner of claim 13, wherein a swirleris coupled to the outer tube within the large-diameter inlet portion toswirl the air through the flow passage.
 20. The burner of claim 13,wherein the means for discharging fuel discharges fuel into thelarge-diameter outlet portion of the outer tube.
 21. The burner of claim13, wherein the inlet end of the large-diameter inlet portion is adaptedto be coupled to a fuel supply to conduct an air-and-fuel mixturethrough the flow passage.
 22. The burner of claim 13, wherein afuel-injection manifold is adapted to the coupled to the inlet end ofthe large-diameter inlet portion of the outer tube, the manifoldincluding a ring defining an air-flow passageway and at least onespoke-like injector tube for discharging fuel through apertures formedin the ring into the air passing through the air-flow passageway.
 23. Aburner comprising means for moving an air flow in sequence through afirst section at a low velocity, a second section at an acceleratingvelocity that is higher than the low velocity, and a third section at ahigh velocity that is higher than the accelerating velocity and meansfor discharging fuel carried in a tube through an aperture formed in aside wall of the tube into the air flow to form a combustibleair-and-fuel mixture therein for discharge into a flame chamber at anoutlet end of the means for moving an air flow.
 24. The burner of claim23, wherein the means for discharging fuel discharges fuel into thethird section.
 25. The burner of claim 23, further including a swirlercoupled to the first section to swirl the air flowing through the first,second, and third sections.
 26. The burner of claim 23, wherein themeans for discharging fuel discharges fuel into the second section. 27.The burner of claim 26, further including a swirler coupled to the firstsection to swirl the air flowing through the first, second, and thirdsections.
 28. The burner of claim 23, wherein the means for dischargingfuel discharges fuel into the first section.
 29. The burner of claim 28,further including a swirler coupled to the first section to swirl theair flowing through the first, second, and third sections.
 30. A burnercomprising an outer tube defining a flow passage and including an inletportion having a large diameter, an outlet portion having a smalldiameter being smaller than the large diameter of the inlet portion, anda nozzle portion interconnecting an outlet end of the inlet portion andan inlet end of the outlet portion to establish the flow passage, aninlet end of the large-diameter inlet portion being adapted to becoupled to an air-and-fuel mixture supply to conduct an air-and-fuelmixture flowing through the flow passage to an outlet end of the outertube, and a swirler positioned within the inlet portion of the outertube.
 31. The burner of claim 30, further comprising a small-diameterinner tube being positioned to lie in the flow passage of the outer tubeand having a diameter that is smaller than the small diameter of theouter tube and a bluff-body flame holder being coupled to the inner tubeand extending beyond an outlet end of the outlet portion of the outertube.
 32. The burner of claim 31, further comprising a swirler coupledto the outer tube to swirl the air flowing through the flow passage. 33.The burner of claim 32, wherein the swirler is positioned within theinlet portion of the outer tube.
 34. The burner of claim 32, wherein theswirler is positioned within the nozzle portion of the outer tube. 35.The burner of claim 30, further comprising a swirler coupled to theouter tube to swirl the air flowing through the flow passage.
 36. Aburner comprising an outer tube defining a flow passage and including aninlet portion having a large diameter, an outlet portion having asmaller diameter than the large diameter of the inlet portion, and anozzle portion interconnecting an outlet end of the inlet portion and aninlet end of the outlet portion and converging toward the outlet portionto establish the flow passage, an inlet end of the large-diameter inletportion being adapted to be coupled to an air-and-fuel mixture supply toconduct an air-and-fuel mixture flowing through the flow passage to anoutlet end of the outer tube, a small-diameter inner tube beingpositioned to lie in the flow passage of the outer tube, the inner tubehaving an inlet end adapted to be coupled to a fuel supply, the innertube having a diameter that is smaller than the small diameter of theouter tube, and the inner tube being formed to include a plurality offuel-injection holes positioned within the small-diameter outlet portionof the outer tube to conduct fuel flowing through the inner tube intothe air-and-fuel mixture flowing through the flow passage, and a flameholder coupled to a flame outlet end of the inner tube, the flame holderbeing positioned adjacent to the outlet end of the outer tube.
 37. Aburner comprising an outer tube defining a flow passage and including alarge-diameter inlet portion having a large diameter, a small-diameteroutlet portion having a small diameter being smaller than the largediameter of the large-diameter inlet portion, and a nozzle portioninterconnecting an outlet end of the inlet portion and an inlet end ofthe outlet portion to establish the flow passage, an inlet end of thelarge-diameter inlet portion being adapted to be coupled to anair-and-fuel mixture supply to conduct an air-and-fuel mixture flowingthrough the flow passage to an outlet end of the outer tube and afuel-injection manifold coupled to the inlet end of the large-diameterinlet portion of the outer tube, the manifold including a ring definingan air-flow passageway and at least one spoke-like injector tubeextending between the ring and the outer tube for discharging fuelthrough apertures formed in the ring into the air passing through theair-flow passageway.
 38. A burner comprising an outer shell defining arectilinear flow passage and including an inlet portion having a largevolume, an outlet portion having a smaller volume than the large volumeof the inlet portion, and a nozzle portion interconnecting an outlet endof the inlet portion and an inlet end of the outlet portion to establishthe rectilinear flow passage, an inlet end of the large-volume inletportion being adapted to be coupled to an air supply to conduct airflowing through the flow passage to an outlet end of the outer shell anda supplemental tube being positioned to extend perpendicularly throughthe flow passage of the outer shell, the supplemental tube having afirst end adapted to be coupled to a fuel supply and the supplementaltube being formed to include a fuel-injection hole to conduct fuelflowing through the supplemental tube into the air flowing through therectilinear flow passage.
 39. A burner comprising a first tube defininga first flow passage and including an inlet portion having a largediameter an outlet portion having a smaller diameter than thelarge-diameter inlet portion, and a nozzle portion interconnecting theinlet portion and the outlet portion to establish the first flow passagean inlet end of the large-diameter inlet portion being adapted to becoupled to an air supply to conduct air through the first flow passagetowards an outlet end of the first tube, a second tube defining a secondflow passage and being positioned to lie in the first flow passage, thesecond tube having an inlet end adapted to be coupled to a fuel supplyand being formed to include at least one fuel-injection hole to conductfuel flowing through the second flow passage into the air flowingthrough the first flow passage, the second tube also having an outletend adapted to be coupled to a flame holder to prevent fuel from exitingthe outlet end of the second tube, the flame holder being positionedadjacent to the outlet end of the outer tube, a third tube defining athird flow passage and being positioned to lie in the second flowpassage, an inlet end of the first tube being adapted to be coupled toan oxygen supply to conduct oxygen through the third flow passagetowards the outlet end of the second tube, the third tube extendingthrough the flame holder to conduct the oxygen out of the outlet end ofthe second tube, and a fourth tube defining a fourth flow passage andbeing positioned to lie in the third flow passage, an inlet end of thefirst tube being adapted to be coupled to a secondary fuel supply toconduct secondary fuel through the fourth passage towards the outlet endof the second tube, the fourth tube extending through the flame holderto conduct the secondary fuel out of the outlet end of the second tube.40. A burner comprising a first tube defining a first flow passage andincluding an inlet portion having a large diameter, an outlet portionhaving a smaller diameter than the large-diameter inlet portion, and anozzle portion interconnecting the inlet portion and the outlet portionto establish the first flow passage, an inlet end of the large-diameterinlet portion being adapted to be coupled to an air supply to conductair through the first flow passage towards an outlet end of the firsttube, a second tube defining a second flow passage and being positionedto lie in the first flow passage, the second tube having an inlet endadapted to be coupled to a fuel supply and begin, formed to include atleast one fuel-injection hole to conduct fuel flowing through the secondflow passage into the air flowing through the first flow passage, thesecond tube also having an outlet end adapted to be coupled to a flameholder to prevent fuel from exiting the outlet end of the second tube,the flame holder being positioned adjacent to the outlet end of theouter tube, and a third tube defining a third flow passage and beingpositioned to lie in the second flow passage, an inlet end of the firsttube being adapted to be coupled to a waste-gas supply to conductwaste-gas through the third flow passage towards the outlet end of thesecond tube, the third tube extending through the flame holder toconduct the waste-gas out of the outlet end of the second tube.
 41. Aburner assembly comprising a burner having an outer tube including aninlet portion and an opposite outlet portion, the outer tube beingadapted to be coupled to a furnace chamber such that the inlet portionof the outer tube is positioned to lie outside the furnace chamber andthe outlet portion is positioned to lie inside the furnace chamber and ashort refractory block being coupled to the outlet portion of the outertube inside the furnace chamber and being shorter than the outletportion such that the outlet portion extends beyond the refractory blockfurther into the furnace chamber.
 42. A burner comprising an outer tubedefining a flow passage and including an inlet portion having a largeeffective cross-sectional area, an outlet portion having a smalleffective cross-sectional area being smaller than the effectivecross-sectional area of the inlet portion, and a nozzle portioninterconnecting an outlet end of the inlet portion and an inlet end ofthe outlet portion to establish the flow passage, an inlet end of theinlet portion being adapted to be coupled to an air supply to conductair flowing through the flow passage to an outlet end of the outer tube,an inner tube being positioned to lie in the flow passage of the outertube, the inner tube having an inlet end adapted to be coupled to a fuelsupply and an opposite downstream end, the inner tube having aneffective cross-sectional area that is smaller than the effectivecross-sectional area of the outer tube, and the inner tube being formedto include a fuel-injection hole positioned to lie in spaced-apartrelation to the downstream end to conduct fuel flowing through the innertube into the air flowing through the flow passage to establish acombustible air-and-fuel mixture within the flow passage, and a flameholder coupled to the downstream end of the inner tube, the flame holderbeing positioned to lie outside the flow passage defined by the outertube to complete the mixing of the fuel with the air as the combustibleair-and-fuel mixture exits the outlet end of the outer tube to produce alow-emission flame attached to the flame holder upon ignition of thecombustible air-and-fuel mixture.